1. Field of the Invention
The present invention relates to a light emitting device and a method for improving luminescence efficiency of a light emitting device. More particularly, the present invention relates to a light emitting diode (LED) and a method for improving luminescence efficiency thereof.
2. Description of Related Art
Recently, since luminescence efficiency of LEDs has been constantly upgraded, application of fluorescent lamps and/or incandescent bulbs is gradually replaced with LEDs in some areas, such as scanning light source which requires high speed response, back or front light source of a liquid crystal display (LCD), car dashboard illumination, traffic signs and general illumination devices. Compared with the traditional bulbs, LED has absolute advantages. For example, an LED is physically compact, long lasting, low voltage/current driven, durable, mercury free (pollution free) and with high emissivity (power saving) etc.
FIG. 1 is a schematic cross-sectional view showing a conventional LED. Referring to FIG. 1, the conventional LED 100 comprises a substrate 110, a semiconductor layer 120, a first electric conductive layer 130, a first electrode 140, a second electric conductive layer 150, a second electrode 160 and a passivation layer 170. The first electric conductive layer 130 is disposed on the substrate 110. The semiconductor layer 120 is disposed on the first electric conductive layer 130. The semiconductor layer 120 comprises an N-type doped semiconductor layer 122, a light emitting layer 124 and a P-type doped semiconductor layer 126 from the bottom to the top. The first electrode 140 penetrates through the region of the N-type doped semiconductor layer 122 which the light emitting layer 124 is not disposed thereon and electrically connects the first electric conductive layer 130. The second electric conductive layer 150 is disposed on the P-type doped semiconductor layer 126. The second electrode 160 is disposed on the second electric conductive layer 150. The passivation layer 170 covers on the semiconductor layer 120 and exposes at least a portion of the first electrode 140 and of the second electrode 160.
With reference to FIG. 1, when current is applied between the first electrode 140 and the second electrode 160 of the LED 100, the light emitting layer 124 will emit light when current passes. However, due to the smooth surface of the passivation layer 170, the light emitted from a light emitting layer 124 inclines producing a total reflection on the surface of the passivation layer 170. Consequently, the emissivity of the LED 100 will be reduced. Besides, SiO2 and SiNx are often used as the material of the conventional passivation layer 170, which has poor light transmission. Thus, the luminescence efficiency of the LED 100 can not be improved.
Furthermore, the heat dissipating capacity of the prior art material, such as SiO2 and SiNx, used in the passivation layer 170 is poor. In addition, the substrate 110 composed of sapphire has bad heat dissipating capacity. Therefore, not only the performance of the LED 100 is inefficient, but also the lifetime of the LED is shortened resulting from the poor heat dissipating capacity.
Therefore, how to improve the luminescence efficiency and at the same time extend the lifetime of LEDs becomes an imminent issue to be solved.